The present invention relates to a process for the preparation of relief copies by imagewise irradiating a positive-working, light-sensitive copying material which is composed of a layer support and a radiation-sensitive layer, and washing out the irradiated layer areas. The radiation-sensitive layer contains a compound which upon irradiation splits off acid and a compound which has at least one acid-cleavable C--O--C group.
Copying materials of this general type are known and are, e.g., described in U.S. Pat. Nos. 3,779,778 and 4,101,323; in German Pat. No. 27 18 254 and in German Offenlegunsschriften Nos. 28 29 512 and 28 29 511. The acid-cleavable compounds contained therein are orthocarboxylic acid derivatives, monomeric or polymeric acetals, enolethers or acyliminocarbonates. The radiation-sensitive compounds contained therein which split off acid predominantly are organic halogen compounds, especially s-triazines which have been substituted by halogenomethyl groups.
The radiation-sensitivity, and in particular the sensitivity to long-wave ultraviolet light, of these compositions is considerably increased compared with other industrially employed positive systems, such as, e.g., o-quinone diazides.
It has been found, however, that the radiation sensitivity of the copying materials based on acid-cleavable compounds to a certain extent depends on the time which elapses between the exposure and developing, i.e., the maximum practical sensitivity is only obtained when some minutes are allowed to elapse between exposure and developing. In case of a storage time of about 3 minutes between exposure and developing, the practical sensitivity to light typically increases by up to 1.5 wedge steps, i.e., about 70% compared with the value which is measured when the time interval between exposure and developing is only 15 seconds. When this time interval is extended to about 5-40 minutes, constant sensitivity values are obtained.
In order to obtain high constant sensitivity values, it is therefore advisable to wait after the exposure for at least 5-10 minutes before developing is carried out. Thus the time saved due to the high sensitivity to light is largely lost during the processing.
In the case of continuously proceeding treatment processes such as, e.g., the preparation of printing forms, it is possible to store the exposed plates in a holder, from which they are removed and developed after a predetermined time. If a large number of plates are to be prepared, this delay occurs only once, at the beginning, and is of minor importance, but such a holder is of a complicated constructional design and requires much floor space. Moreover, in case of a continuous-operation service, there is always the danger that exposure deficiencies are discovered only after a large number of plates have been subjected to this incorrect exposure. Besides, it is also desired to reduce this time required in advance when single plates are processed, so that the total treatment can be carried out within the time otherwise required.